Just Another Way

auth.js

const allowedStatus = ['artist', 'client', 'shop'];

const authorizeUser = (req, res, next) => {
  let Model;
  
  if(!allowedStatus.includes(req.body.status)){
    res.status(422).json("Credentials don't match");   
  }
  
  if(req.body.status === 'artist'){
    Model = Artist;
  }
  
  if(req.body.status === 'client'){
    Model = Client;
  }
  
  if(req.body.status === 'shop'){
    Model = Shop;  
  }
  
  Model.findOne({username: req.body.username})
    .exec((err, user) => {
      if(err) return res.status(401);
      const token; //token conversion logic here
      req.token = token;
    });
}

const signup = (req, res) => {
  //just do your biz, status type has already been validated and assigned to req. 
}